Convertible tread for a radial truck or trailer tire

ABSTRACT

A radial ply pneumatic tire  10  for trucks or trailers has a tread  12  with at least three circumferential grooves  20,22  extending circumferentially to divide the tread  12  into at least four parts and lateral grooves  30,31,32,33,34  extending across the at least four parts of the tread to form at least four rows  1,2,3,4  and  5  of circumferentially separated blocks  40.  The circumferential grooves  20,22  have average groove widths of W 1  for a depth between the radially outer surface of the adjacent blocks and a groove depth that is less than 60% of the non-skid depth D, the circumferential grooves having a second average groove width W 2  at the groove depth less than 60% of the non-skid depth D, the second average groove width W 2  being less than 50% of W 1 . The lateral grooves  30,34  in each shoulder row  15  have a first average groove width W 3  and a groove depth that is less than 60% of the non-skid depth D. The lateral grooves  31,32,33  in the central rows  2,3  and  4  have a first average length L 1  for a depth between a radially outer surface of the adjacent blocks and a groove depth that is less than 60% of the non-skid depth D, the lateral grooves in the central rows having a second average length L 2  at the groove depth that is less than 60% of the non-skid depth D, the second average length L 2  being greater than the first average length L 1 . The tie  10  when new, provides a tread  12  for the drive axle position and when less than 60% worn, the tread pattern changes to one ideally suited for the trailer wheel position.

TECHNICAL FIELD

This invention relates to the field of heavy duty radial pneumatic tiresfor trucks and truck trailers.

BACKGROUND ART

In many areas of the world, heavy duty vehicles such as trucks must havesuperior traction performance on the drive axle positions of thevehicle. Tires having sufficiently aggressive tread patterns with blockelement type tread patterns seem to provide the best tractionperformance.

As the tread becomes about half worn these block elements can lose someof their traction capability or performance. Ideally, the drive axletires would be replaced at that time. Unfortunately, for the vehicleowner this means that the tread still had a useful half life remainingif it could be used in a less traction sensitive wheel position.Unfortunately, the original tread pattern when worn was not conducive tosuch applications such as trailer tires.

The need was apparent that tread patterns must change or convert whenworn partially to a different pattern if they were to be successfullyused in other wheel positions.

The use of convertible truck tires which start out having one block typetread pattern and as the tire reached a certain level of wear changed toa rib type tire was taught in U.S. Pat. No. 4,223,712 wherein less thanfull depth inclined transverse grooves would disappear after the tirewas 30% to 70% worn.

A later U.S. Pat. No. 4,732,195 issued Mar. 22, 1988, taught that thetransverse grooves should disappear toward the axially outside of thetread so that the separated blocks change into continuous ribs from theaxially outside to the inside of the tread in sequence as the treadwears.

In U.S. Pat. No. 4,854,384 issued Aug. 8, 1989, taught a block typetread pattern which is also converted to a rib type tire when the tireis worn in the range of 35% to 75% of the maximum depth. The tread has aplurality of circumferentially continuous zig-zag and straight grooveswhich have a total width equal to 15-35% of the entire tread width whenthe tire is new and 10-25% of the entire tread width when the tire isworn and the transverse grooves completely disappear to form a ribpattern consisting solely of zig-zag longitudinal grooves.

In these patents the tread pattern simply was converted from blockelements to ribs by the loss of the transverse groove.

These rib type tires generally will exhibit uniform wear and can be usedwhere traction must be achieved by zig-zagging the longitudinal grooves.Historically, such tires have relatively poor braking traction when usedon wet or snowy roads.

The present invention in a unique way converts a block element treadpattern for drive axles of trucks when the tire is half worn to acombination of shoulder ribs and central rows of larger block elementswhich can exhibit substantially superior wet traction capability thanthe prior art ribbed type tires.

DISCLOSURE OF THE INVENTION

A radial ply pneumatic tire 10 for trucks or trailers having a tread 12with at least three longitudinal grooves 20,22 that extendcircumferentially to divide the tread 12 into at least four parts isdisclosed, more preferably 5 parts divided by four longitudinal grooves20,22. The tread 12 has lateral grooves 30,31,32,33,34 extending acrossthe at least four parts of the tread 12 to form at least four rows1,2,3,4,5 of circumferentially separated blocks 40 including twoshoulder rows 1,5 and at least two, preferably three central rows 2,3,4.Each block 40 has a radially outer surface 42.

The longitudinal grooves 20,22 have an average width W₁ between theradially outer surface 42 of the tread blocks 40 extending to a depth ofless than 60% of the total tread depth D thereafter the average width ofthe longitudinal grooves 20,22 narrows to less than 50% of W₁ and thelateral grooves 30,34 in each shoulder row extends to a depth of lessthan 60% of the total tread depth.

The lateral grooves in the central rows 31,32,33 have an average lengthof L₁ between the radially outer surface 42 of the tread blocks 40 to adepth of less than 60% of the total depth D, thereafter the lateralgrooves 31,32,33 average length increases to greater than L₁, preferablyto L₁+50% of W₁ in length. The average groove width of the lateralgrooves 31,32,33 in the central rows equals W₃, the width W₃ beingmeasured between the radially outer surface 42 of the block 40 to adepth of less than 60% of the total depth D, thereafter the lateralgrooves narrow to an average width W₄ of less than 80% of W₃ for theremainder of the groove depth, preferably less than 50% of W₃.

These changes in average groove widths W₁ of the longitudinal grooves20,22 and the average groove widths W₃ of the central row lateralgrooves 31, 32, 33 to less than 50% W₁ and less than 80% W₃,respectively, preferably occurs at the same depth. Furthermore, it ispreferred that the lateral grooves 30,34 in the shoulder rows 1,5 ceaseat the same depth wherein the longitudinal and the lateral groovesnarrow.

It is further believed important that the longitudinal and lateralgrooves change average width at 40% or more of the total depth D of thetread 12.

In the preferred embodiment tire each block element has at least onesipe 51,52,53 and a portion of the sipe 51,52,53 remains in the tread 12after the width of the longitudinal 20,22 and lateral grooves 31,32,33narrows due to the tire 10 being worn.

The tire 10 when new preferably has a net-to-gross ratio of less than70% which increases by at least 10% at the depth of the tread where theaverage groove widths narrow, preferably increasing by almost 20%.

Definitions

“Aspect ratio” of the tire means the ratio of its section height (SH) toits section width (SW) multiplied by 100% for expression as apercentage.

“Asymmetric tread” means a tread that has a tread pattern notsymmetrical about the centerplane or equatorial plane EP of the tire.

“Circumferential” means lines or directions extending along theperimeter of the surface of the annular tread perpendicular to the axialdirection.

“Equatorial plane (EP)” means the plane perpendicular to the tire's axisof rotation and passing through the center of its tread.

“Footprint” means the contact patch or area of contact of the tire treadwith a flat surface at zero speed and under normal load and pressure.

“Groove” means an elongated void area in a tread that may extendcircumferentially or laterally about the tread in a straight, curved, orzigzag manner. Circumferentially and laterally extending groovessometimes have common portions. The “groove width” is equal to the treadsurface area occupied by a groove or groove portion, the width of whichis in question, divided by the length of such groove or groove portion;thus, the groove width is its average width over its length. Grooves maybe of varying depths in a tire. The depth of a groove may vary aroundthe circumference of the tread, or the depth of one groove may beconstant but vary from the depth of another groove in the tire. If suchnarrow or wide grooves are of substantially reduced depth as compared towide circumferential grooves which they interconnect, they are regardedas forming “tie bars” tending to maintain a rib-like character in thetread region involved.

“Inboard side” means the side of the tire nearest the vehicle when thetire is mounted on a wheel and the wheel is mounted on the vehicle.

“Lateral” means an axial direction.

“Net contact area” means the total area of ground contacting elementsbetween defined boundary edges divided by the gross area between theboundary edges as measured around the entire circumference of the tread.

“Net-to-gross ratio” means the total area of ground contacting treadelements between the lateral edges around the entire circumference ofthe tread divided by the gross area of the entire tread between thelateral edges.

“Non-directional tread” means a tread that has no preferred direction offorward travel and is not required to be positioned on a vehicle in aspecific wheel position or positions to ensure that the tread pattern isaligned with the preferred direction of travel. Conversely, adirectional tread pattern has a preferred direction of travel requiringspecific wheel positioning. “Axial” and “axially” means lines ordirections that are parallel to the axis of rotation of the tire.

“Non-Skid Depth” means the full depth on total depth of the tread fromthe surface of the tread to the bottom of the deepest groove.

“Outboard side” means the side of the tire farthest away from thevehicle when the tire is mounted on a wheel and the wheel is mounted onthe vehicle.

“Radial” and “radially” means directions radially toward or away fromthe axis of rotation of the tire.

“Rib” means a circumferentially extending strip of rubber on the treadwhich is defined by at least one circumferential groove and either asecond such groove or a lateral edge, the strip being laterallyundivided by full-depth grooves.

“Sipe” means small slots molded into the tread elements of the tire thatsubdivide the tread surface and improve traction.

“Tread element” or “traction element” means a rib or a block element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a developed fragmentary plan view of a preferred embodimenttire having a block type tread pattern according to the invention.

FIG. 2 is a plan view of the tire of FIG. 1 after tread wear has causedthe conversion of the tread pattern to the two shoulder ribs withcentral rows of enlarged blocks.

FIGS. 3-7 are cross sectional views taken of the various longitudinaland lateral grooves.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1 there is shown a fragmentary plan view of thepreferred embodiment tire 10. The tire 10 has a tread 12 having at leastthree longitudinal grooves 20,22 dividing the tread into at least fourparts. As illustrated, the tread 12 of the preferred tire has four suchlongitudinal grooves 20,22 dividing the tread into five parts. The tread12 has a plurality of lateral grooves 30,31,32,33,34 extending acrosseach part and dividing the parts forming at least four, as illustratedfive rows 1,2,3,4,5, of circumferentially separated blocks 40.

Each block 40 has at least one sipe 51,52,53. Several of the transversegrooves 30,31,33 and 34 have a sipe 55. Numerous short lateral sipes 54are closely spaced and positioned on a radially inner portion of theblock elements 40 adjacent shoulder grooves 22.

A plurality of stone penetration protectors 60 are shown extending fromthe base of the circumferential grooves 20,22. These features helpprotect the tire from having small stones or rocks from penetratingthrough the base of the grooves 20,22, and thus exposing the underlyingreinforcing belt structure.

As illustrated, the longitudinal grooves 20,22 are straight and as shownin cross sectional views of FIGS. 3 and 4 have full depths D (excludingtread wear indicators and the stone penetration protectors 60). The fulldepth D is commonly referred to as the non-skid depth of the tread.

Each groove 20, 22 has a first average groove width W₁ before the tread12 is worn as shown in FIG. 1, the adjacent tread blocks 40 to a depthless than 60% of the total depth D. Thereafter, the width of the grooves20,22 substantially narrow to a width of less than 50% of W₁, as shownabout 30% of W₁.

With reference to the cross-sectional views of FIGS. 5-7, thecross-sectional shape of the various lateral grooves 30,31,32,33,34 areshown. In each case, the lateral groove has a first average widthdesignated W₃ which is measured between the radially outer surfaces 42of the adjacent blocks 40 to a depth of less than 60% of the depth D.

In the shoulder rows 1 and 5, the lateral grooves stop at the groovedepth of less than 60% of the depth D. As shown, these grooves extendradially inward from the tread surface at least 40% of D. Beyond thatdepth the groove ceases and the shoulder rows become solid shoulderribs.

With reference to the blocks 40 of the central rows 2,3 and 4, thelateral grooves 31,32,33 each has an average groove width designated W₂,W₂ being between the radially outer surface 42 of the block 40 to adepth of less than 60% of D and preferably to a depth of at least 40% ofD. Thereafter, a majority of the lateral grooves 31,32,33 preferably allthe lateral grooves narrow to a width less than W₂ for its respectivelateral groove, more preferably narrowing by about 80% less than theoriginal W₂.

By measuring along the radially outer surface 42 of each block 40adjacent to a particular lateral groove one can determine the averagelength of the lateral groove, each such average length being designatedL₁. As the tread 12 wears to a location wherein the longitudinal grooves20,22 narrow the lateral grooves in the central rows 2,3 and 4 actuallyincrease in length. As shown in FIG. 2 the length increases from L₁ to alength of L₁+greater than or equal to 50% of W₁. As shown each lateralgroove 31,32,33 increased by an amount equal to about ⅔ of W₁ in thecentral rows.

Further inspection of FIG. 2 shows that the net-to-gross ratio of thepreferred tire increased to about 85% from the original or new tirenet-to-gross ratio of about 68%. It is believed that the new tirenet-to-gross ratio should be at or below 70% and after wearing down thetread depth to between 40% and 60% of D, the narrowing or elimination ofthe grooves should result in at least a 10% preferably 15% or moreincrease in the net-to-gross contact ratio.

This increase in tread rubber in contact with the road surface greatlyretards the rate of tread wear. The fact that the lateral grooves31,32,33 in the central rows are effectively much longer means that thetractive efficiency is also maintained at a higher rate than the earliermentioned prior art rib type tires. Furthermore, the wet tractionperformance of the inventive tire is enhanced by the combination of thelong lateral grooves and the straight longitudinal grooves. The use ofstraight longitudinal grooves 20,22 along with a plurality of blockshaving lengthened lateral grooves markedly increases the tractiveperformance of the worn tire. The generous use of lower level sipes 54,and 55, along with maintaining portions of sipes 51,52,53, alsopositively contribute to traction performance.

The sipes 54 are as shown in dashed lines of cross-sectional view inFIG. 3. The sipes originate in the area of the groove walls adjacent thecircumferential groove 22. The sipes are at or slightly below theintersection of the groove wall and the radially outer surface 42 of theblock element 40 and extend radially inwardly to a location at, orslightly above, the base of the groove, preferably above the base of thegroove or above the stone penetration protectors 60. As the tread 12wears these sipes 54, which are originally partially hidden in thegrooves 22, are exposed. When the tread reaches the transition point ofwear, the sipes 54 are then fully laterally exposed and they becomefunctionally important. At that level of tread wear the tire tread isconverted to a pattern useful on free rolling axles of vehicles such astrailers. These sipes 54 help in retarding the initiation of riverwearthat is a common occurrence in the blocks or ribs of such tires in thelocation designated in rows 2 and 4 at the axially outer portionsadjacent grooves 22.

Although the block elements 40 of FIG. 1 and FIG. 2 are shown almostaligned in a circumferential direction, it is believed that it may bedesirable to stagger the rows 1,2,3,4,5 so that the axially adjacentblocks are more circumferentially offset.

Another important feature of this invention is the groove 24 shown inFIG. 1 as a line 24. As the tread wears, this narrow straightcircumferential groove, which is initially not readily observable,becomes observable when the drive axle tread pattern is fully worn, thatis as shown in FIG. 2 at the 40% to 60% worn condition earlierdiscussed. At that level of wear, the trailer pattern is fully exposedas well as this narrow groove 24. This narrow groove functions as apressure distribution groove which helps reduce or eliminate theoccurrence of chamfer wear common in the free rolling axle position oftires used on truck trailers.

Another important feature is that each of the lateral grooves have theirfirst and second ends 30A,30B, 31A,31B,32A,32B,33A,33B,34A,34B orientedperpendicular to the longitudinal direction and having one or morestraight inclined portions oriented about 45° or more relative to thelongitudinal direction. This configuration insures that a large portionof the lateral grooves are perpendicularly oriented to the direction offorward travel. This greatly improves wet and dry braking traction.

As can be appreciated from the above description, this tire according tothe invention does not compromise the traction performance of theshoulder ribs and central block type pattern in the 60% worn condition.Great attention has been given to enhancing both the traction and thewear properties of the worn tread. The conversion for one aggressivedrive axle type tread to a trailer type tread having an aggressivecombination of shoulder ribs and enlarged central row blocks makes thetire according to the present invention superior to those of the priorart.

What is claimed is:
 1. A radial ply pneumatic tire for trucks andtrailers having a tread with a non-skid depth D, at least threecircumferential grooves extending circumferentially to divide the treadinto at least four parts, lateral grooves extending across the at leastfour parts of the tread to form at least four rows of circumferentiallyseparated blocks, including two shoulder rows, and at least two centralrows, each block having a radially outer surface, the tire characterizedby: the circumferential grooves having a first average groove width W₁for a depth between the radially outer surface of the adjacent blocksand a groove depth that is less than 60% of the non-skid depth D, thecircumferential grooves having a second average groove width W₂ at thegroove depth that is less than 60% of the non-skid depth D, the secondaverage groove width W₂ being less than 50% of the first average groovewidth W₁, the lateral grooves in each shoulder row having a firstaverage groove width W₃ and a groove depth of less than 60% of thenon-skid depth D, the lateral grooves in the central rows having a firstaverage length of L₁ for a depth between the radially outer surface ofthe adjacent blocks and a groove depth that is less than 60% of thenon-skid depth D, the lateral grooves in the central rows having asecond average length L₂ at the groove depth that is less than 60% ofthe non-skid depth D, the second average length L₂ being greater thanthe first average length L₁, the lateral grooves in the central rowshaving a second average groove width W₄ at the groove depth that is lessthan 60% of the non-skid depth D, and wherein the second average groovewidth W₂ of the circumferential grooves and the second average groovewidth W₄ of the lateral grooves in the central rows occur at the sameradial depth, the lateral grooves in the shoulder rows cease at the samedepth that the second groove widths W₂ and W₄ occur when the tread isworn to the location at which widths W₂ and W₄ occur, thereby formingtwo shoulder ribs and at least two central rows of circumferentiallyspaced blocks.
 2. The radial ply pneumatic tire for trucks and trailersof claim 1 wherein the tire has an initial net-to-gross ratio of lessthan 70%, which increases by at least 10% at the groove depth at whichthe second average groove widths W₂ and W₄ occur.
 3. The radial plypneumatic tire for trucks and trailers of claim 1 wherein the secondaverage length L₂ equals the first average length L₁+ at least 50% ofthe first average groove width W₁.
 4. The radial ply pneumatic tire fortrucks and trailers of claim 1 wherein each block element has at leastone sipe, a portion of the sipe remains in the tread after the width ofthe circumferential and lateral grooves narrows due to the tire beingworn to the location at which W₂ and W₄ occur.
 5. The radial plypneumatic tire for trucks and trailers of claim 1 where the central rowlateral grooves in the 60% or more worn condition are orientedperpendicular to the direction of travel over more than half of theirlength.
 6. The radial ply pneumatic tire for trucks and trailers ofclaim 1 wherein the circumferential grooves axially inward and adjacentto one of the shoulder rows have groove walls having numerous lateralextending sipes that appear as the tread wears.
 7. The radial plypneumatic tire for trucks and trailers of claim 1 wherein at a locationaxially outward of the shoulder row of blocks and radially inwardthereof lies a narrow circumferentially straight groove that appears asthe tread wears.